Why Refracting Telescopes Reached Their Maximum Practical Size
with the Yerkes 40"  in 1897:

Refractors use lenses, which refract, or bend light.  They also split
light into its component colors, much as prisms do: this is called
chromatic aberration.

To minimize chromatic aberration, refractors must have long focal lengths.

-> Refractor tubes are very long, with no way to fold them up
(as with reflectors)

-> Long tubes (Yerkes 40-inch's tube is 80 feet long!)

-> Tall piers (Yerkes: 40 feet high)

-> Domes like great cathedrals

-> HUGE COSTS

(Also bad "dome-effect" seeing, from huge dome trapping lots of air)

Also: 

- Lenses bend out of shape over time, due to their own weight

- Mirrors don't, since they can be supported from the back

Also: 

- Glass from which lenses are made must be PERFECT: no bubbles or cracks,
perfectly transparent. 

- Mirrors are more forgiving.  Only one surface matters.

-> Mirrors only need to be polished and aluminized (formerly silvered)
on one side; lenses need both sides, and often more, if multi-element
(to reduce chromatic aberration).

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Many of these considerations also apply to amateur telescopes,
e.g. refractors are very expensive, per aperture!

-> BEWARE of cheap, junky, "department store" telescopes:
with plastic lenses and spindly, shaky little mounts!


BINOCULARS and a good tripod are often the best "first" telescope.

A lot also depends on you:

see Sky & Telescope's page on "Choosing Your First Telescope", at

http://www.skypub.com/tips/telescopes/firstscope.html


BEWARE ALSO: Magnification is the most hyped benchmark.

                 focal length of telescope
Magnification =  _________________________
                 focal length of eyepiece

Largely irrelevant: just put in a different eyepiece!

Besides, because of seeing, maximum usable magnification ~ 150x